JP2012122130A - High-strength steel plate with excellent formability, warm working method, and warm-worked automotive part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel plate that is more excellent in ductility while maintaining strength of not less than 980 MPa class.SOLUTION: The high-strength steel plate includes, in percentages by mass, 0.05-0.3% C, 1-3% Si, 0.5-3% Mn, not more than 0.1% P (including 0%), not more than 0.01% S (including 0%), 0.001-0.1% Al, and 0.002-0.03% N. The remainder has a component composition comprising iron and impurities, and has a structure including, in terms of an area ratio relative to the entire structure, 50-90% bainitic ferrite, not less than 3% retained austenite (γ), 10-50% martensite plus the γabove, and not more than 40% ferrite (including 0%). The γhas a concentration C (Cγ) of 0.5-1.2 mass%, and not less than 0.3% of γexists surrounded by the martensite.

Description

  The present invention relates to a high-strength steel sheet excellent in formability, a warm working method using the same, and a warm-worked automobile part. The high-strength steel sheet of the present invention includes a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.

  Thin steel plates used for automobile frame parts are required to have high strength in order to realize collision safety and fuel efficiency improvement. Therefore, it is required to ensure press formability while increasing the strength of the steel sheet to 980 MPa class or higher. It is known that in a high-strength steel sheet of 980 MPa class or higher, it is effective to use steel utilizing the TRIP effect to achieve both high strength and formability (for example, see Patent Document 1).

Patent Document 1 discloses a high-strength steel plate containing bainite or bainitic ferrite as a main phase and containing retained austenite (γ _R ) in an area ratio of 3% or more. However, this high-strength steel sheet has a tensile strength at room temperature of 980 MPa or more and does not reach an elongation of 20%, and further improvement in mechanical properties (hereinafter also simply referred to as “characteristics”) is required.

  On the other hand, since there is a limit to the formability of TRIP steel sheet in cold forming, the TRIP effect is further effectively expressed by increasing the elongation by warm working at 100 to 400 ° C for further improvement of elongation. Techniques have been proposed (see Non-Patent Document 1 and Patent Document 2).

As shown in Table 2 of Patent Document 2, the elongation around 200 ° C. is improved to 23% in the 1200 MPa class by making γ _R having a carbon concentration of 1 mass% or more present in the structure mainly composed of bainitic ferrite. is made of. However, since these steel sheets have a comparatively narrow temperature range in which mechanical properties can be ensured, it is difficult to stably form them during press forming.

JP 2003-19319 A JP 2004-190050 A

Koichi Sugimoto, Takeshi Hoshi, Takeya Sakaguchi, Akihiko Nagasaka, Takahiro Kashima, “Warm Formability of Ultra High Strength Low Alloy TRIP Type Bainitic Ferritic Steel”, Iron and Steel, 2005, Vol. 91, No. 2, p. 34-40

  The present invention has been made by paying attention to the above circumstances, and its purpose is to secure a strength of 980 MPa class or higher and a high-strength steel sheet having superior ductility (press workability) and warm working using the same. It is an object of the present invention to provide a method and an automotive part warm worked by the method.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.3%
Si: 1-3%
Mn: 0.5-3%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-90%
Residual austenite: 3% or more,
Martensite + the above retained austenite: 10 to 50%,
Ferrite: 40% or less (including 0%)
Having an organization including
The residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass,
This retained austenite is a high-strength steel sheet with excellent formability, characterized by the presence of 0.3% or more of martensite.

The invention described in claim 2
Ingredient composition further
Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
The high strength steel sheet having excellent formability according to claim 1, wherein B: one or more of 0.00001 to 0.01%.

The invention according to claim 3
Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The high-strength steel sheet having excellent formability according to claim 1 or 2, wherein the REM contains one or more of 0.0001 to 0.01%.

The invention according to claim 4
It is a warm processing method of the high strength steel plate characterized by processing the high strength steel plate according to any one of claims 1 to 3 within 3600 seconds after heating to 200 to 400 ° C.

The invention described in claim 5
The automobile part processed by the method according to claim 4, wherein a region where the true strain applied during processing is 0.05 or more and a region where it is less than 0.05 are mixed, and the region where the true strain is maximum The difference in yield stress between the first part and the smallest part is 200 MPa or less.

According to the present invention, bainitic ferrite: 50 to 90%, retained austenite: 3% or more, martensite + the above retained austenite: 10 to 50%, ferrite: 40% or less (0) The residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass, and the residual austenite surrounded by martensite A high-strength steel sheet that ensures elongation while maintaining a strength of 980 MPa or more in a wide temperature range, a warm working method using the same, and It is now possible to provide processed auto parts.

It is a cross-sectional structure photograph of this invention steel plate and a comparative steel plate.

As described above, the present inventors pay attention to a TRIP steel sheet containing bainitic ferrite having a substructure (matrix) with a high dislocation density and residual austenite (γ _R ), similar to the above-described conventional technology, Further studies have been made to further improve the stretchability while ensuring the strength.

As a result, (1) after ensuring the strength by partially introducing martensite into the structure, (2) γ _R having a carbon concentration of 0.5 to 1.2% by mass is 3% or more by area ratio. by including, enhance elongation by TRIP effect, (3) further, among the above gamma _R, 0.3% or more gamma _R at an area ratio, without let contact the matrix of bainitic ferrite, hard By covering it with martensite, it is difficult to apply strain to the γ _R during plastic deformation, suppressing processing-induced transformation at the early stage of deformation, and making it easier to cause processing deformation-induced transformation at the later stage of deformation. It has been found that curing can be realized in a wide range, and the present invention has been completed based on this finding.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on the structure of TRIP steel as in the above prior art. In particular, the steel sheet contains a predetermined amount of martensite and has a carbon concentration of 0.5 to 1.2% by mass. It differs from the above prior art in that it contains 3% or more of γ _R in area ratio, and further, among the above γ _R , those surrounded by martensite are present in an area ratio of 0.3% or more. is doing.

<Bainitic ferrite: 50-90%>
“Bainitic ferrite” in the present invention has a substructure having a lath-like structure with a high dislocation density in the bainite structure and is free of carbides in the structure. It is clearly different, and is also different from the polygonal ferrite structure having a substructure with little or no dislocation density, or a quasi-polygonal ferrite structure having a substructure such as fine subgrains (Japan Iron and Steel Institute Fundamental Study Group) Issued “Steel Bainite Photobook-1”).

  Thus, the balance of strength and formability can be improved by using bainitic ferrite having a uniform and fine structure, high ductility, high dislocation density and high strength as the parent phase.

In the steel sheet of the present invention, the amount of the bainitic ferrite structure needs to be 50 to 90% (preferably 60 to 90%, more preferably 70 to 90%) in terms of area ratio with respect to the entire structure. is there. This is because the effect of the bainitic ferrite structure is effectively exhibited. Note that the amount of the bainitic ferrite structure is determined by the balance with γ _R, and it is recommended that the amount be controlled appropriately so that desired characteristics can be exhibited.

<Contains 3% or more of retained austenite (γ _R ) in area ratio with respect to the entire structure>
γ _R is useful for improving the total elongation, and in order to effectively exhibit such action, the area ratio is 3% or more (preferably 5% or more, more preferably 10% or more) with respect to the entire structure. It is necessary to exist. However, if it is present in a large amount, the stretch flangeability deteriorates, so it is preferably 20% or less.

<Martensite + Retained austenite (γ _R ): 10-50%>
For securing strength, but introduces some martensite in the tissue, since the moldability amount of martensite is too large can not be secured, 10% total area fraction of martensite + gamma _R for all tissues It is limited to 50% or less (preferably 12% or more, more preferably 16% or more).

<Ferrite: 40% or less (including 0%)>
Since ferrite is a soft phase and effective in increasing ductility, it may be introduced in an area ratio of 40% or less that can guarantee strength. Preferably it is 30% or less.

<C concentration of residual austenite (γ _R ) (Cγ _R ): 0.5 to 1.2% by mass>
Cγ _R is, γ _R at the time of processing is an indicator that affects the stability of the transformation to martensite. When C gamma _R is too low, gamma for _R is unstable, after stressing, since the deformation-induced martensitic transformation occurs prior to plastic deformation, not bulging property can be obtained. On the other hand, when the C gamma _R is too high, gamma _R becomes too stable, since the addition of machining work-induced martensitic transformation does not occur, not too bulging property can be obtained. To obtain a sufficient stretch forming property, C gamma _R is required to be 0.5 to 1.2 mass%. Preferably it is 0.6-1.1 mass%.

<Residual austenite (γ _R ) surrounded by martensite is 0.3% or more in area ratio with respect to the entire structure>
Part of gamma _R by covering martensite rigid, the remainder of the gamma _R was expressed relatively early TRIP effect of deformation, the gamma _R is deformed initially surrounded by martensite to the gamma _R The concentration of strain is prevented, and the TRIP effect due to the processing-induced martensitic transformation is expressed in the later stage of deformation. By doing in this way, since the TRIP effect is manifested in a wide strain range in cold forming, high elongation is obtained, and in warm forming, γ _R having an appropriate stability can be present in a wide temperature range. The temperature range where elongation can be obtained is expanded.

Said to express the TRIP effect in a wide range, the gamma _R surrounded by martensite, should be present in area ratio 0.3% of the total tissue.

Steel sheet of the present invention, to the extent the tissue only (martensite and / or bainitic ferrite, mixed structure of polygonal ferrite and gamma _R) may be composed of, not impairing the effects of the present invention, other As the heterogeneous structure, bainite may be included. Although this structure can inevitably remain in the manufacturing process of the steel sheet of the present invention, the smaller the number, the better. It is recommended to control the area ratio to 5% or less, more preferably 3% or less with respect to the entire structure. Is done.

[Each phase area ratio, gamma C concentration of _R (C gamma _R), and each method for measuring the area ratio of gamma _R surrounded by martensite]
Here, each phase area ratio, C concentration of γ _R (Cγ _R), and will be described the method for measuring the area ratio of gamma _R surrounded by martensite.

The area ratio of the microstructure in the steel sheet is measured by repeller corrosion of the steel sheet, and by observing with a light microscope (magnification 1000 times), for example, the white area is defined as “martensite + retained austenite (γ _R )”. did.

Incidentally, gamma C concentration area ratio and gamma _R of the _R (C gamma _R), after grinding to 1/4 the thickness of the steel sheet was measured by X-ray diffraction method from the chemical polishing (ISIJ Int.Vol.33 (1933), No. 7, p. 776). The area ratio of the ferrite was determined by corroding the steel plate with nital and identifying an agglomerated white region having an equivalent circle diameter of 5 μm or more as ferrite by optical microscope observation (magnification 400 times). Further, after the area ratio of other structures such as pearlite was identified by optical microscope observation (magnification 1000 times), portions other than “martensite + residual austenite (γ _R )”, “ferrite” and “other structures” were The area ratio was calculated as tick ferrite.

Area ratio of gamma _R surrounded by martensite was determined as follows. First, SEM is combined with an OIM (Orientation Imaging Microscopy ™) analysis system, EBSD (Electron Back Scattering Diffraction Pattern) measurement is performed at a pitch of 0.2 μm, and the FCC phase, BCC phase, and BCC phase are adjacent crystal grains. And grain boundaries having an orientation difference of 15 ° or more are mapped. In this mapping, identifying the mapped region as FCC phase is defined as gamma _R. Further, a region where the area of the grain boundary having an orientation difference of 15 ° or more in the BCC phase is 5 measurement points or less and a region that could not be analyzed as the FCC phase or the BCC phase are defined as martensite and identified. The γ _R completely surrounded by the martensite thus identified was identified and defined as γ _R surrounded by the martensite, and the area ratio was determined.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.05-0.3%
C is an essential element for obtaining a desired main structure (bainitic ferrite + martensite + γ _R ) while ensuring high strength, and 0. It is necessary to add 05% or more (preferably 0.10% or more, more preferably 0.15% or more). However, if it exceeds 0.3%, it is not suitable for welding.

Si: 1-3%
Si is an element that effectively suppresses the generation of carbides by decomposition of γ _R. In particular, Si is useful as a solid solution strengthening element. In order to exhibit such an action effectively, it is necessary to add Si 1% or more. Preferably it is 1.1% or more, More preferably, it is 1.2% or more. However, if Si is added in excess of 3%, the formation of bainitic ferrite + martensite structure is hindered, the hot deformation resistance is increased, and the welds are easily embrittled. It also has an adverse effect on the surface properties of the film, so the upper limit is made 3%. Preferably it is 2.5% or less, More preferably, it is 2% or less.

Mn: 0.5 to 3%
In addition to effectively acting as a solid solution strengthening element, Mn also exerts an effect of promoting transformation and promoting the formation of bainitic ferrite + martensite structure. Furthermore, it is an element necessary for stabilizing γ and obtaining a desired γ _R. In order to exhibit such an action effectively, it is necessary to add 0.5% or more. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, when it is added in excess of 3%, adverse effects such as slab cracking are observed. Preferably it is 2.5% or less, More preferably, it is 2% or less.

P: 0.1% or less (including 0%)
P is inevitably present as an impurity element, but is an element that may be added to ensure desired γ _R. However, when it exceeds 0.1%, secondary workability deteriorates. More preferably, it is 0.03% or less.

S: 0.01% or less (including 0%)
S is also an element unavoidably present as an impurity element, forms sulfide inclusions such as MnS, and becomes a starting point of cracking and deteriorates workability. Preferably it is 0.01% or less, More preferably, it is 0.005% or less.

Al: 0.001 to 0.1%
Al is an element which is added as a deoxidizer and effectively suppresses the generation of carbides by decomposition of γ _{R in} combination with Si. In order to exhibit such an action effectively, it is necessary to add 0.001% or more of Al. However, even if added excessively, the effect is saturated and is economically wasteful, so the upper limit is made 0.1%.

N: 0.002 to 0.03%
N is an unavoidable element, but forms a precipitate when combined with carbonitride-forming elements such as Al and Nb, and contributes to strength improvement and microstructure refinement. And austenite grain coarsening the N content is too low, as a result, the aspect ratio for gamma _R which elongated lath structure becomes mainly increases. On the other hand, if the N content is too high, casting becomes difficult with low carbon steel such as the material of the present invention, and therefore the production itself cannot be performed.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and unavoidable impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired. .

Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
B: One or more elements of 0.00001 to 0.01% These elements are useful elements for strengthening steel and are effective in stabilizing γ _R and securing a predetermined amount. In order to effectively exhibit such an action, Mo: 0.01% or more (more preferably 0.02% or more), Cu: 0.01% or more (more preferably 0.1% or more), Ni : 0.01% or more (more preferably 0.1% or more) and B: 0.00001% or more (more preferably 0.0002% or more) are recommended. However, even if Cr is added in an amount of 3%, Mo is added in an amount of 1%, Cu and Ni are added in an amount of more than 2%, and B is added in an amount exceeding 0.01%, the above effect is saturated, which is economically wasteful. More preferably, Cr is 2.0% or less, Mo is 0.8% or less, Cu is 1.0% or less, Ni is 1.0% or less, and B is 0.0030% or less.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: One or more of 0.0001 to 0.01% These elements are effective elements for controlling the form of sulfide in steel and improving workability. Here, examples of the REM (rare earth element) used in the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above-mentioned action, Ca and Mg are each added to 0.0005% or more (more preferably 0.0001% or more), and REM is added to 0.0001% or more (more preferably 0.0002% or more). It is recommended to do. However, even if Ca and Mg are added in an amount of 0.01% and REM is added in excess of 0.01%, the above effects are saturated, which is economically wasteful. More preferably, Ca and Mg are 0.003% or less, and REM is 0.006% or less.

[Warm processing method]
The steel sheet of the present invention is excellent in elongation and deep drawability even at room temperature, and may be cold worked for forming into parts, but after being heated to an appropriate temperature between 200 and 400 ° C. It is particularly recommended to work within 3600 s (more preferably within 1200 s).

Elongation and deep drawability can be maximized by processing before the decomposition of γ _R occurs under temperature conditions where the stability of γ _R is optimal.

〔Auto parts〕
The automobile parts processed by the warm processing method are excellent in elongation and deep drawability, and in particular, the true strain applied during the warm processing is 0.05 or more and less than 0.05. A region in which the difference in yield stress between the region having the maximum true strain and the region having the minimum true strain is 200 MPa or less is recommended.

steel sheet containing gamma _R is typically a low yield ratio and a high rate of work hardening in a low strain region. For this reason, in the region where the applied strain is small, the strength after applying the strain, in particular, the strain dependence of the yield stress, becomes very large. When a part is formed by press working, the amount of strain applied varies depending on the part, and there is a region where strain is hardly applied partially. For this reason, a large strength difference may occur between a region where machining is performed and a region where machining is not performed in the component, and a strength distribution may be formed in the component. When such an intensity distribution exists, deformation and buckling occur due to the yielding of the low-strength region, so that the part having the lowest strength is rate-determined.

The reason why the yield stress is low in the steel containing γ _R is thought to be that when γ _R is introduced, martensite formed at the same time introduces mobile dislocations in the surrounding matrix during transformation. Therefore, if this dislocation movement is prevented even in a region where the amount of processing is small, the yield stress can be improved and the component strength can be increased. In order to suppress the movement of movable dislocations, it is effective to heat the material to eliminate the movable dislocations or to stop it by strain aging such as solute carbon, which can increase the yield stress.

Therefore, when a steel sheet containing γ _R is heated to an appropriate temperature between 200 ° C. and 400 ° C. and press-formed (warm processing), the yield strength is increased even in a portion with a small strain, and the strength distribution in the part is reduced. As a result, the component strength can be improved.

  Specifically, even in a component having a low strain region in which a region where the true strain applied during the press molding (warm processing) is 0.05 or more and a region less than 0.05 is mixed, A part having a difference in yield stress between the part having the maximum true strain and the part having the minimum true strain of 200 MPa or less is more suitable as an automobile part because the strength distribution in the part is small and the part strength is high. Become.

  Next, the preferable manufacturing method for obtaining the said steel plate of this invention is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention is manufactured by hot-rolling a steel material satisfying the above component composition, followed by cold rolling, followed by heat treatment, but in order to surround a part of γ _R with martensite. The manufacturing conditions may be set based on the following concept. That is, by controlling the bainite transformation in the austemper to an appropriate stage to control the carbon concentration in the untransformed austenite to an appropriate level, and keeping the size of the untransformed austenite coarse, cooling after austempering a portion of the untransformed austenite martensite transformation during, by a mechanism such as the untransformed austenite to martensite remains, during cooling after austempering completion, martensite is formed in a manner to surround the gamma _R . At this time many be gamma _R content rate of martensitic transformation in the in cooling the carbon content is too low in austenite can not be secured. Meanwhile, since so most of untransformed austenite remains as gamma _R and the carbon content is too high, gamma _R content surrounded by the martensite is reduced. Then, in order to increase the size of the gamma _R, it is necessary to keep the coarse initial tissue.

[Hot rolling conditions]
Therefore, the hot rolling finish temperature (rolling end temperature, FDT) is set to 900 to 1000 ° C., and the coiling temperature is set to 600 to 700 ° C., which is higher than the conventional one, thereby making the structure of the hot rolled material coarser than before. By doing so, the structure formed in the subsequent heat treatment process becomes coarse, and as a result, the size of γ _R also increases.

[Cold rolling conditions]
In addition, by reducing the cold rolling ratio during cold rolling to 10 to 30% (more preferably 10 to 20%), the recrystallized structure during heating in the subsequent annealing step is roughened and further cooled. The reverse transformation structure in is roughened.

[Heat treatment conditions]
Regarding heat treatment conditions, soaking in austenitic (γ + α) two-phase region or γ single-phase region, soaking at a predetermined cooling rate and supercooling, followed by a predetermined time at the supercooling temperature A desired tissue can be obtained by holding and austempering. It should be noted that plating or further alloying treatment may be performed without significantly degrading the desired structure and within the range not impairing the action of the present invention.

  Specifically, the following heat treatment conditions are recommended. That is, in order to austenitize the cold-rolled material after the cold rolling, 0.6Ac1 + 0.4Ac3 or more (preferably 0.5Ac1 + 0.5Ac3 or more) 950 ° C. which is a (γ + α) two-phase region or a γ single-phase region. After holding for 1800 s or less (preferably 900 s or less) in the temperature range of 930 ° C. or less (preferably 900 s or less), 3 ° C./s or more (preferably 5 ° C./s or more, more preferably 10 ° C./s or more, particularly preferably At an average cooling rate of 20 ° C./s or more), it is rapidly cooled to a temperature range of 350 to 500 ° C. (preferably 380 to 480 ° C., more preferably 420 to 460 ° C.) and supercooled (to the above (1) The heat treatment conditions are the same as those in the case.) The austempering treatment is performed at this quenching stop temperature (supercooling temperature) for 10 to 100 s (preferably 20 to 60 s), and then cooled to room temperature. .

  In addition, it is not restricted to the said recommended heat processing conditions, For example, the structure | tissue of this invention can be obtained also on the following heat processing conditions. That is, in order to austenitize the cold-rolled material after the cold rolling, 0.6Ac1 + 0.4Ac3 or more (preferably 0.5Ac1 + 0.5Ac3 or more) 950 ° C. which is a (γ + α) two-phase region or a γ single-phase region. After holding for 1800 s or less (preferably 900 s or less) in the temperature range of 930 ° C. or less (preferably 900 s or less), 3 ° C./s or more (preferably 5 ° C./s or more, more preferably 10 ° C./s or more, particularly preferably At an average cooling rate of 20 ° C./s or more), it is rapidly cooled to a temperature range of 350 to 500 ° C. (preferably 380 to 480 ° C., more preferably 420 to 460 ° C.) and supercooled (to the above (1) The heat treatment conditions are the same as in the case.) After the austempering treatment by holding for 10 to 100 s (preferably 20 to 60 s) at this quenching stop temperature (supercooling temperature) (to the above ( ) Is the same heat treatment conditions as in.), 480-600 ° C. (preferably after alloying treatment by time reheat 1~100s in a temperature range of four hundred and eighty to five hundred and fifty ° C.), cooled to room temperature.

[Examination of composition and production conditions affecting mechanical properties of high-strength steel sheet]
In this example, the influence of the mechanical properties of the high-strength steel sheet when the component composition and manufacturing conditions were changed was investigated. After vacuum-melting the test steel consisting of each component composition shown in Table 1 to form a slab with a plate thickness of 30 mm, the slab was hot-rolled under each production condition shown in Table 2 and cold-rolled. Heat treatment was applied. Specifically, the slab is heated to 1200 ° C., hot-rolled to a sheet thickness tmm at a rolling finish temperature (FDT) T1 ° C., then placed in a holding furnace at a coiling temperature T2 ° C., and air-cooled. Simulated board winding. Then, it cold-rolled by the cold rolling rate r%, and was set as the cold-rolled board of plate thickness 12mm. The cold-rolled material was heated to a soaking temperature T3 ° C. at 10 ° C./s, held at that temperature for 90 s, then cooled at a cooling rate R4 ° C./s, and held at a supercooling temperature T5 ° C. for t5 seconds. Thereafter, it was cooled with air, or held at a supercooling temperature T5 ° C. for t5 seconds, and further held at a holding temperature T6 ° C. for t6 seconds, and then cooled with air.

The steel sheet thus obtained, by a measuring method described in the section of [Description of the Invention, each phase area ratio, C concentration of γ _R (Cγ _R), and, in martensite the area ratio of the enclosed gamma _R was measured. The area ratio of gamma _R surrounded by martensite was measured using those incorporating the OIM analysis system TSL manufactured in SEM made JEOL (model number JSM-5410).

  In addition, in order to evaluate the cold and warm mechanical properties of the steel sheet, tensile strength (TS), elongation [total elongation (EL)], and at room temperature and 300 ° C., respectively, in the following manner: Deep drawability [limit drawing ratio (LDR)] was measured.

  TS and EL were measured using a JIS No. 5 test piece by a tensile test. The strain rate in the tensile test was 1 mm / s. In addition, LDR was formed by deep-drawing a test piece having a diameter of 80 to 140 mm using a cylindrical mold having a die diameter of 53.4 mm, a punch diameter of 50.0 mm, and a shoulder R of 8 mm and a wrinkle-reducing pressure of 9.8 kN. Measured.

  These results are shown in Tables 3 and 4.

  As shown in these tables, steel no. 1 to 3, 9 to 18, and 26 to 33 are all steels satisfying the range of the component composition of the present invention, and are manufactured under recommended manufacturing conditions. It was a steel sheet, and both the room temperature characteristics and the warm characteristics met the judgment criteria, and a high-strength steel sheet excellent in formability was obtained.

  On the other hand, Steel No. Nos. 4 to 8 and 19 to 25 are comparative steel plates that do not satisfy at least one of the component composition and the structure requirement defined in the present invention, and the room temperature characteristics and the warm characteristics do not satisfy the judgment criteria.

Incidentally, the distribution state of γ _{R in} the structure of the steel plate of the present invention (steel No. 27) and the comparative steel plate (steel No. 19) is illustrated in FIG. FIG. 1 shows the results of EBSD measurement. The portion indicated by a gray or black small hexagon is γ _R , the portion indicated by a white small hexagon is martensite, and the portion indicated by a white wide area is a matrix bay. Nitic ferrite. From this figure, in the comparative steel plate (steel No. 19), there are many γ _Rs connected to each other, basically being in contact with the bainitic ferrite of the matrix, In the steel sheet of the present invention (steel No. 27), it can be seen that γ _R is finely divided and is often embedded (enclosed) in a fine martensite phase.

[Examination of appropriate temperature range for warm working]
Next, in order to investigate the appropriate temperature range when warming the steel sheet of the present invention, the steel No. 27, the heating temperature was sequentially changed between 150 to 450 ° C., and the warm characteristics were measured. The results are shown in Table 5. The results at a temperature of 300 ° C. in the same table indicate the steel No. in Table 4 above. 27 warm characteristics are shown again.

  As shown in this table, while the temperature of less than 200 ° C. or over 400 ° C. does not meet the criteria for warm characteristics, the temperature between 200 ° C. and 400 ° C. satisfies the criteria for warm characteristics. It was found that excellent warm formability was exhibited in a wide temperature range.

[Examination of the effect of processing temperature on strength variations in parts]
Furthermore, in order to investigate the influence of the processing temperature on the variation in strength due to the strain distribution in the part obtained by processing the steel sheet of the present invention, 27, at each of room temperature and 300 ° C., with no processing giving true strain at all (true strain 0%), or after applying processing to give 5% true strain, again at room temperature A tensile test was performed to measure the yield stress (YS). The results are shown in Table 6.

  As shown in the same table, the parts obtained by warm working the steel sheet of the present invention have less variation in yield stress due to differences in the amount of work in the parts than parts obtained by cold working, It was confirmed that the component strength was improved.

Claims (5)

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.3%
Si: 1-3%
Mn: 0.5-3%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-90%
Residual austenite: 3% or more,
Martensite + the above retained austenite: 10 to 50%,
Ferrite: 40% or less (including 0%)
Having an organization including
The residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass,
A high-strength steel sheet excellent in formability, characterized in that 0.3% or more of the retained austenite is surrounded by martensite. Ingredient composition further
Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
The high-strength steel sheet having excellent formability according to claim 1, wherein B: one or more of 0.00001 to 0.01%. Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The high-strength steel sheet having excellent formability according to claim 1 or 2, comprising REM: 0.0001 to 0.01% of one or more.   A warm-working method for a high-strength steel sheet, wherein the high-strength steel sheet according to any one of claims 1 to 3 is processed within 3600 s after being heated to 200 to 400 ° C.   The automobile part processed by the method according to claim 4, wherein a region where the true strain applied during processing is 0.05 or more and a region where it is less than 0.05 are mixed, and the region where the true strain is maximum A difference in yield stress between the first part and the smallest part is 200 MPa or less.